Fibre Optic Cables

Fibre Optic cables consists of a core and a cladding layer, selected for total internal reflection due to the difference in the refractive index between the two. In practical fibres, the cladding is usually coated with a layer of acrylate polymer or polyimide. This coating protects the fibre from damage but does not contribute to its optical waveguide properties. Individual coated fibres (or fibres formed into ribbons or bundles) then have a tough resin buffer layer and/or core tube(s) extruded around them to form the cable core. Several layers of protective sheathing, depending on the application, are added to form the cable. Rigid fiber assemblies sometimes put light-absorbing (“dark”) glass between the fibres, to prevent light that leaks out of one fibre from entering another. This reduces cross-talk between the fibre optic cables, or reduces flare in fiber bundle imaging applications.

For indoor applications, the jacketed fibre is generally enclosed, with a bundle of flexible fibrous polymer strength members like aramid (e.g. Twaron or Kevlar), in a lightweight plastic cover to form a simple Power cables. Each end of the cable may be terminated with a specialized Fibre Optic connector to allow it to be easily connected and disconnected from transmitting and receiving equipment.

For use in more strenuous environments, a much more robust cable construction is required. In loose-tube construction the fibre optic is laid helically into semi-rigid tubes, allowing the cable to stretch without stretching the fibre optic itself. This protects the fibre from tension during laying and due to temperature changes.

A critical concern in outdoor cabling is to protect the fibre optic cables from contamination by water. This is accomplished by use of solid barriers such as copper tubes, and water-repellent jelly or water-absorbing powder surrounding the fibre optic cables.

Finally, the cable may be armored to protect it from environmental hazards, such as construction work or gnawing animals. Undersea cables are more heavily armored in their near-shore portions to protect them from boat anchors, fishing gear, and even sharks, which may be attracted to the electrical power that is carried to power amplifiers or repeaters in the fibre optic cables.

Modern cables come in a wide variety of sheathings and armor, designed for applications such as direct burial in trenches, dual use as power lines, installation in conduit, lashing to aerial telephone poles, submarine installation, and insertion in paved streets.

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